The Cambrian explosion, also called the Cambrian radiation or Cambrian diversification, was a time period that began about 538.8 million years ago during the Cambrian period of the early Paleozoic era. During this time, a rapid increase in complex life forms occurred, and most major animal groups began to appear in the fossil record. This event lasted for about 13 to 25 million years and led to the splitting of most modern animal groups. Other groups of organisms also diversified significantly during this time.
Before the Cambrian explosion, most life forms were simple, made up of single cells or small groups of cells, sometimes forming colonies. As life diversified more quickly, the variety of living things became much more complex, resembling today’s ecosystems. Nearly all animal groups that exist today first appeared during this period, including the earliest chordates.
History and significance
William Buckland noted in the 1840s that fossils appeared quickly in the "Primordial Strata." Charles Darwin discussed this mystery in his 1859 book On the Origin of Species, calling it a major challenge for his theory of natural selection. Scientists have long puzzled over the sudden appearance of Cambrian animals without clear ancestors. Three main questions remain: Did many complex organisms diversify quickly during the early Cambrian? What caused this rapid change? And what does it reveal about the origin of animal life? Answers are hard to find because the fossil record is incomplete, and evidence relies on limited data and chemical clues in Cambrian rocks.
The first Cambrian fossils discovered were trilobites, described by Edward Lhuyd in 1698. William Buckland later realized that a major shift in the fossil record occurred near the base of the Cambrian. Nineteenth-century geologists like Adam Sedgwick and Roderick Murchison used Cambrian fossils to date rock layers and define the Cambrian and Silurian periods. By 1859, some geologists believed the lowest Silurian stratum showed Earth’s first life, though others, like Charles Lyell, disagreed. Darwin called the sudden appearance of trilobites without earlier fossils "of the gravest nature" for his theory. He suggested that earlier seas had many creatures, but their fossils were missing due to gaps in the record. In his sixth edition, he emphasized this problem further.
Charles Walcott, a paleontologist, studied the Burgess Shale and proposed a missing time period called the "Lipalian," during which Cambrian ancestors might have evolved. Later evidence showed life existed 3,850 million years ago, with stromatolites in Australia and more complex cells in rocks from 1,400 million years ago. Ediacaran fossils from 580 to 543 million years ago showed large, multicelled organisms unlike modern life. In 1948, Preston Cloud suggested a sudden evolutionary burst in the Cambrian, but no clear link to later Cambrian life was found until the 1970s.
Interest in the "Cambrian explosion" grew when Harry Whittington and colleagues reanalyzed Burgess Shale fossils in the 1970s. They found complex creatures like Marrella, which was an arthropod but not part of any known group, and Opabinia and Wiwaxia, which seemed unrelated to modern animals. Stephen Jay Gould’s 1989 book Wonderful Life popularized these findings. Both Whittington and Gould suggested that most modern animal groups appeared quickly during the Cambrian, influencing ideas like Darwin’s "tree of life" and the theory of punctuated equilibrium, which describes long periods of little change followed by rapid evolution.
Other studies argue that complex animals evolved before the Cambrian. Radiometric dating of Cambrian rocks has only recently become available, and matching rock layers across continents remains difficult. The start of the Cambrian was dated to 542 million years ago in 2004, revised to 541 million years ago in 2012, and adjusted again to 538.8 million years ago in 2022.
Some theories link the Cambrian explosion to the final stages of the Gondwana supercontinent’s formation, which followed the breakup of Rodinia and the opening of the Iapetus Ocean. The largest Cambrian animal communities were near Gondwana, spanning from low northern to high southern latitudes. By the Cambrian’s middle and later periods, continents like Laurentia, Baltica, and Siberia drifted apart.
Fossils of whole organisms are the most informative evidence, but fossilization is rare. Most fossils are destroyed by erosion or metamorphism before they can be studied. The fossil record is incomplete, especially for older times. Biases also exist, as certain environments and body parts are more likely to be preserved. For example, only hard parts like mollusk shells often survive, while soft-bodied animals decay quickly. Although 30+ animal phyla exist today, two-thirds have never been found as fossils.
The Cambrian period has many lagerstätten, which preserve soft tissues. These sites, like the Chengjiang shale in China and the Burgess Shale in Canada, allow scientists to study internal anatomy, unlike conventional fossils that only show hard parts. However, lagerstätten are rare and limited to specific environments where soft-bodied organisms are quickly buried. They do not represent typical living conditions, and Precambrian lagerstätten remain poorly studied.
The sparse fossil record means many organisms may exist long before they appear in the fossil record.
Explanation of key scientific terms
A phylum is the highest level in the Linnaean system for grouping living things. Phyla are categories that group animals based on their general body structures. Even though animals may look very different on the outside, they are placed in the same phylum if they share similar internal structures and how they develop. For example, spiders and barnacles are both in the phylum Arthropoda, but earthworms and tapeworms are in different phyla even though they look similar. As scientists learn more through chemical and genetic testing, some phyla are changed or removed from classification systems.
A phylum is not a basic part of nature, like the difference between electrons and protons. It is a broad category in a system designed to classify living organisms. This system is not perfect, even for modern animals. Different books may list different numbers of phyla because scientists often disagree about how to classify many worm-like species. Since the system is based on living organisms, it does not work well for extinct ones.
The idea of stem groups was created to describe evolutionary relatives of living groups. These groups are based on scientific theories. A crown group includes living animals that are closely related, along with their most recent common ancestor and all its descendants. A stem group includes animals that split off from the lineage before the crown group’s ancestor. This is a relative idea. For example, tardigrades are a crown group on their own, but some scientists think they are also a stem group related to arthropods.
The term "triploblastic" means an animal has three layers of cells formed early in development, starting from a single cell. The inner layer becomes the digestive tract (gut), the outer layer becomes the skin, and the middle layer becomes muscles and internal organs (except the gut). Most living animals are triploblastic. Exceptions include sponges (Porifera) and jellyfish and sea anemones (Cnidaria).
Bilaterians are animals that have right and left sides at some point in their life. This means they also have top and bottom surfaces and clearly defined front and back ends. All known bilaterians are triploblastic, and all known triploblastic animals are bilaterians. Living echinoderms, such as sea stars and sea urchins, appear radially symmetrical (like wheels) as adults, but their larvae show bilateral symmetry. Some early echinoderms may have had bilateral symmetry. Sponges and jellyfish are radially symmetrical and not triploblastic. The ancestor shared by bilaterians and cnidarians is thought to have had a larva with bilateral symmetry.
The term "coelomate" refers to animals with a body cavity (coelom) that holds internal organs. Most phyla studied in the Cambrian explosion debate are coelomates, such as arthropods, annelid worms, molluscs, echinoderms, and chordates. Priapulids are an important exception because they are not coelomates. All known coelomates are triploblastic bilaterians, but some triploblastic bilaterians, like flatworms, do not have a coelom. Their organs are surrounded by unspecialized tissues instead.
Precambrian life
Changes in the number and variety of certain fossils have been seen as signs of attacks by animals or other organisms. Stromatolites, short, pillar-like structures made by groups of tiny microbes, were a major part of the fossil record from about 2,700 million years ago. However, their numbers and variety dropped sharply after about 1,250 million years ago. This decline is thought to be caused by animals that grazed on or dug through the sediment.
Before the Cambrian period, the ocean was filled with small fossils called acritarchs. This term describes almost any small fossil with an organic outer layer, such as egg cases from tiny animals or resting structures from green algae. Acritarchs first appeared around 2,000 million years ago, and their numbers, variety, size, and complexity increased greatly around 1,000 million years ago. Their spiny shapes in the last 1 billion years may suggest a need for protection from predators. Other small organisms from the Neoproterozoic era also show signs of defenses against predators. Studies of how long different groups of organisms have existed support the idea that predation increased around this time.
In general, the fossil record shows that these lifeforms appeared very slowly during the Precambrian period, with many cyanobacterial species forming much of the sediment.
At the start of the Ediacaran period, many acritarch species, which had not changed much for hundreds of millions of years, became extinct. They were replaced by a variety of larger, new species that did not last long. This growth in diversity, the first major one in the fossil record, was followed by a group of unfamiliar, large fossils called the Ediacara biota. These organisms thrived for 40 million years until the start of the Cambrian period. Most of the Ediacara biota were at least a few centimeters long, much larger than earlier fossils. These organisms formed three distinct groups, growing larger and more complex over time.
Many of these organisms were unlike any that came before or after, looking like discs, mud-filled bags, or quilted mattresses. One paleontologist suggested that the most unusual ones should be classified as a separate group, called Vendozoa.
At least some of these organisms may have been early forms of animal groups central to the debate about the Cambrian explosion. Some have been compared to early mollusks (like Kimberella), echinoderms (like Arkarua), and arthropods (like Spriggina, Parvancorina, and Yilingia). However, scientists disagree about how to classify these fossils because they lack features that help classify modern organisms, such as clear similarities to living species. Still, there is strong evidence that Kimberella was a bilaterian animal with three tissue layers. These organisms are important in understanding whether the Cambrian explosion was sudden or gradual. If some Ediacara biota were early ancestors of modern animal groups, the "explosion" seems less abrupt than if they were unrelated and quickly replaced by the modern animal kingdom.
Traces of organisms moving on and under the microbial mats that covered the Ediacaran seafloor are preserved from about 565 million years ago. These traces were likely made by organisms similar in shape, size, and movement to earthworms. The burrowers themselves have not been found as fossils, but their need for a head and tail suggests they had bilateral symmetry, which would make them bilaterian animals. These organisms likely fed above the sediment surface but had to burrow to avoid predators.
Cambrian life
Trace fossils, such as burrows, help scientists understand what kinds of life existed in the past. These fossils show that life became more varied at the start of the Cambrian period, with animals quickly moving into freshwater environments, just as they did in the oceans.
Fossils called "small shelly fauna" have been found worldwide and date from just before the Cambrian to about 10 million years after its start. These fossils include a mix of types, such as spines, armor plates, tubes, sponge-like animals, and tiny shells similar to those of brachiopods and snail-like mollusks. Most of these fossils are only 1 to 2 millimeters long.
Although small, these fossils are much more common than complete fossils of the organisms that made them. They help fill in the time gap between the start of the Cambrian and the first well-preserved fossil sites, which are called lagerstätten. This allows scientists to extend the known time ranges of many ancient life groups.
The first cnidarian larvae, named Eolarva, appeared in the Cambrian. However, whether Eolarva is truly a cnidarian larva is still debated. If it is, it would be the earliest evidence of indirect development in animals from the Cambrian period.
Medusozoans, a group of animals, developed complex life cycles with a medusa stage during the Cambrian explosion. This is supported by the discovery of Burgessomedusa phasmiformis.
The earliest trilobite fossils are about 530 million years old. However, trilobites were already diverse and widespread, suggesting they existed for a long time before being found in the fossil record. The first trilobite fossils with hard, mineralized shells mark the beginning of their fossil record, not their actual origin.
Crustaceans, one of the four major modern groups of arthropods, were rare during the Cambrian. Some early Cambrian fossils, like those from the Burgess Shale, were once thought to include crustaceans, but none belong to the group of "true" crustaceans. Instead, evidence of Cambrian crustaceans comes from tiny microfossils. In the Swedish Orsten horizons, Cambrian crustaceans are preserved, but only those smaller than 2 millimeters. This limits the data to juvenile or very small adult organisms.
A more detailed source of information is the organic microfossils found in the Mount Cap formation in Canada. This late Early Cambrian site, dating to about 510 to 515 million years ago, contains microscopic pieces of arthropod cuticle. These fragments were preserved after the rock was dissolved with hydrofluoric acid. The variety of these fossils is similar to modern crustacean communities. Analysis of feeding structures in the formation shows they were highly specialized, unlike most early Cambrian arthropods, which ate messily. These advanced feeding tools likely belonged to a large organism (about 30 cm) and could have led to greater diversity in life forms.
The earliest widely accepted echinoderm fossils appeared in the Late Atdabanian stage. Unlike modern echinoderms, these early Cambrian echinoderms were not all radially symmetrical. These fossils provide clear evidence of the end of the Cambrian explosion or at least show that modern animal groups were already present.
Around the start of the Cambrian (about 539 million years ago), new types of traces, such as vertical burrows like Diplocraterion and Skolithos, and traces made by arthropods like Cruziana and Rusophycus, first appeared. These burrows suggest that worm-like animals developed new behaviors and possibly new physical abilities. Some Cambrian trace fossils indicate that their makers had hard exoskeletons, even if they were not always made of minerals. Both small and large bilaterians (a group of animals) were involved in this expansion into new environments.
Burrows provide strong evidence of complex organisms. They are also easier to preserve than body fossils, so the absence of trace fossils can mean that large, mobile bottom-dwelling animals were not present. Burrows support the idea that the Cambrian explosion was a real increase in life diversity, not just a result of how fossils are preserved.
The earliest skeletal fossils from the Ediacaran and lowest Cambrian (Nemakit-Daldynian) stages include tubes and uncertain sponge spicules. The oldest sponge spicules, made of silica, are about 580 million years old and found in China and Mongolia. However, some scientists question whether these fossils are truly spicules. In the late Ediacaran and early Cambrian, mysterious organisms lived in organic-walled and chitinous tubes, such as Saarina and Sabellidites. These tubes thrived until the start of the Tommotian stage. Mineralized tubes from organisms like Cloudina and Namacalathus appeared near the end of the Ediacaran period. These tubes were often found in carbonate rocks of stromatolite reefs, suggesting they lived in environments less favorable to most animals.
Although these fossils are hard to classify, they are important for two reasons. First, they are the earliest known calcifying organisms, which build shells from calcium carbonate. Second, these tubes helped organisms rise above the seafloor to feed more effectively and, to a lesser extent, protected them from predators and harsh conditions. Some Cloudina fossils have small holes, which may indicate that predators drilled through their shells. This could be evidence of an early "arms race" between predators and prey, a theory that explains the Cambrian explosion.
In the earliest Cambrian, stromatolites (layered rock structures formed by algae) declined, allowing animals to colonize warm-water pools with carbonate sediments. Early Cambrian fossils include anabaritids and Protohertzina, which are the fossilized spines of chaetognaths. Hard structures like shells, sclerites, thorns, and plates first appeared in the uppermost Nemakit-Daldynian stage. These were the earliest examples of halkierids, gastropods, hyoliths, and other rare organisms. The start of the Tommotian stage is marked by a sudden increase in the number and variety of mollusc, hyolith, and sponge fossils, along with skeletal remains of unknown animals, the first archaeocyathids, brachiopods, and to
Stages
The early Cambrian period of diversification lasted about 20 to 25 million years. The high rates of evolution during this time stopped by the start of Cambrian Series 2, which began 521 million years ago, at the same time the first trilobites appeared in the fossil record. Different scientists describe the early Cambrian diversification phases in slightly different ways:
Ed Landing identifies three stages: Stage 1, which began at the Ediacaran-Cambrian boundary, includes the diversification of animals with hard parts and the development of deep, complex burrows; Stage 2, which includes the spread of molluscs and early brachiopods (hyoliths and tommotiids), which likely originated in intertidal areas; and Stage 3, which includes the diversification of trilobites in deeper waters, with little change in intertidal areas.
Graham Budd combines different models to describe the Cambrian explosion, dividing it into four intervals: a "Tube world," lasting from 550 to 536 million years ago, spanning the Ediacaran-Cambrian boundary, dominated by fossils like Cloudina, Namacalathus, and pseudoconodont-like structures; a "Sclerite world," which includes the rise of halkieriids, tommotiids, and hyoliths, lasting until the end of the Fortunian (about 525 million years ago); a "brachiopod world," possibly linked to the unconfirmed Cambrian Stage 2; and a "Trilobite World," which began in Stage 3.
In addition to the shelly fossil record, trace fossils can be divided into five periods: "Flat world" (late Ediacaran), where traces were limited to the surface of sediments; Proterozoic III (after Jensen), with increasing complexity; "pedum world," starting at the base of the Cambrian with the beginning of the T. pedum zone; "Rusophycus world," spanning 536 to 521 million years ago, matching the "Sclerite world" and "Brachiopod world" under the SSF model; and "Cruziana world," which clearly corresponds to the "Trilobite World."
Validity
There is clear evidence that species of Cnidaria and Porifera existed during the Ediacaran period, and some Porifera may have appeared even earlier during the Cryogenian period. Bryozoans, once believed to appear in the fossil record only after the Cambrian period, are now found in Cambrian Age 3 strata in Australia and South China.
Darwin’s fossil record suggested that major metazoan groups appeared over a short time in the early to mid-Cambrian, and this idea was still widely accepted in the 1980s.
However, evidence of Precambrian animals is growing. If Ediacaran Kimberella was a mollusc-like protostome (one of two main groups of coelomates), then the protostome and deuterostome lineages must have split before 550 million years ago (deuterostomes are the other main group of coelomates). Even if Kimberella was not a protostome, it is widely accepted as a bilaterian. Fossils of cnidarians, like jellyfish, found in the Doushantuo lagerstätte show that cnidarians and bilaterians must have diverged more than 580 million years ago.
Trace fossils and predatory borings in Cloudina shells provide more evidence of Ediacaran animals. Some fossils from the Doushantuo formation are thought to be embryos, and one (Vernanimalcula) is considered a bilaterian coelomate, though these interpretations are not universally accepted. Predatory pressure has affected stromatolites and acritarchs for about 1,250 million years.
Some scientists believe evolution sped up ten times faster, but the presence of Precambrian animals suggests the Cambrian explosion was not as sudden as once thought. Statistical analysis shows the Cambrian explosion was not faster than other evolutionary radiations in animal history. However, innovations like hard shells evolved only once in the animal lineage, making a long Precambrian animal history harder to support. The idea that all phyla arose in the Cambrian is incorrect, as representatives of many phyla appeared later in the Phanerozoic. Phyla with hard parts, which dominate the fossil record, may not represent all phyla, as most originated in benthic environments. The fossil record supports a Cambrian explosion limited to benthic animals, with pelagic phyla evolving later.
Ecological complexity among marine animals increased in the Cambrian and later in the Ordovician. Recent research shows that disparity was not as high throughout the Cambrian as once believed, with modern levels of disparity only reached after the early Ordovician.
The diversity of many Cambrian assemblages is similar to today’s, and some believe diversity rose steadily through the Cambrian, stabilizing in the Ordovician. However, this view overlooks the complex branching patterns and rapid evolutionary changes near the Cambrian boundary seen in major animal lineages. Questions about the sudden nature of the Cambrian explosion, raised by Harry Blackmore Whittington, remain unanswered.
Budd and Mann suggested the Cambrian explosion was influenced by a type of survivorship bias called the "Push of the Past." Groups that survive tend to diversify rapidly early in their history, creating the illusion of a sudden evolutionary speed-up, even if diversification rates remained normal overall.
Possible causes
Despite evidence that animals with three tissue layers (triploblastic bilaterians) existed before the Cambrian period, the rate of evolutionary change during the early Cambrian was unusually fast. Scientists have proposed three main reasons for this: changes in the environment, changes in how organisms developed, and changes in how different species interacted. Any explanation must address both when this rapid change occurred and how significant it was.
Earth’s earliest atmosphere had no free oxygen (O₂). The oxygen animals need to breathe today, whether in air or water, was created over billions of years through photosynthesis. Cyanobacteria were the first organisms to develop this ability, slowly adding oxygen to the environment. At first, oxygen levels did not rise much because it reacted quickly with iron and other minerals in rocks and ocean water. Once these reactions reached a limit, oxygen could exist as a gas (O₂) in the atmosphere. Over the next 2.5 billion years, oxygen levels gradually increased.
Higher oxygen levels were linked to greater diversity among eukaryotes (complex cells) long before the Cambrian period. The last common ancestor of all living eukaryotes is believed to have lived about 1.8 billion years ago. Around 800 million years ago, the number and complexity of eukaryote species in the fossil record increased sharply. Before this, eukaryotes likely lived in environments with high sulfur levels. Sulfur can harm the mitochondria (energy-producing parts) of aerobic organisms, limiting oxygen use. When ocean sulfur levels dropped around 800 million years ago, oxygen became more available, supporting greater diversity. Sponges, which evolved earlier, may have helped increase oxygen levels in the ocean, contributing to the Cambrian explosion. Studies of molybdenum isotopes show that rising biodiversity in the Early Cambrian was tied to more oxygen-rich ocean waters, suggesting oxygen played a key role in this evolutionary event.
Low oxygen levels might have limited the growth of large, complex animals. The amount of oxygen an animal can absorb depends on the surface area of its oxygen-absorbing organs (like lungs or gills), while the oxygen needed increases with the animal’s volume. If an animal grows larger, its volume increases faster than its surface area, making it harder to get enough oxygen. However, some Ediacara biota (ancient organisms) reached lengths of several meters millions of years before the Cambrian explosion. Other processes, like building collagen (a protein needed for complex structures) or creating hard exoskeletons, might also require more oxygen. Yet, similar low-oxygen conditions in later periods, like the Phanerozoic, did not stop animal evolution, leading some scientists to question whether oxygen levels directly caused the Cambrian explosion.
Ozone (O₃), which protects Earth from harmful ultraviolet (UV) radiation (wavelengths 200–300 nm), likely existed during the Cambrian period. This ozone layer may have allowed life to develop on land, not just in water.
During the late Neoproterozoic (ending in the early Ediacaran period), Earth experienced massive ice ages, with most of its surface covered in ice. These events may have caused a mass extinction, reducing genetic diversity and creating a "bottleneck." After this, new species may have evolved, leading to the Ediacara biota. However, these ice ages occurred long before the Cambrian period, and it is unclear how they could have caused such diversity. Glaciers may have eroded rocks, depositing nutrients into the ocean and preparing the way for the Cambrian explosion.
New research suggests that volcanic activity along mid-ocean ridges caused a sudden increase in ocean calcium levels, enabling marine organisms to build hard parts like shells. Alternatively, erosion from widespread weathering, such as Powell’s Great Unconformity, may have released ions into the ocean. Calcium levels may also have risen due to erosion of the Transgondwanan Supermountain, whose roots are now found in East Africa.
Phosphorus levels in Ediacaran oceans were lower than in later periods, which might have delayed the development of organisms with phosphatic shells.
Studies from 2014 using boron isotopes found that ocean alkalinity increased until the Early Cambrian. This would have made it easier for organisms to form calcium carbonate, helping them create hard parts like shells and spines.
Some theories suggest that small changes in how animals develop from embryos to adults could lead to major differences in their final forms. Hox genes, for example, control which organs develop in specific body regions. A slight change in which Hox gene is active might turn a limb into an eye. This system allows a wide range of body types to arise from a limited set of genes. However, these theories struggle to explain why such genetic systems alone caused the Cambrian explosion. Evidence from Precambrian animals and genetic data shows that many genes needed for the explosion were already present by the Cambrian period.
A theory focusing on physical development suggests that the rise of simple multicellular life created new conditions where physical processes, previously used by single-celled organisms, could generate complex body structures like layers, segments, and appendages.
Horizontal gene transfer, where genes move between species, may have helped organisms rapidly gain the ability to form hard parts, such as shells. Evidence suggests a key gene for biomineralization was originally transferred from bacteria to sponges.
Other theories focus on interactions between species. Some explain changes in food chains, while others suggest predator-prey "arms races" or coevolution. These ideas help explain why diversity and complexity increased quickly but do not fully explain why this happened.
Relationship with the Great Ordovician Biodiversification Event
After an extinction event at the boundary between the Cambrian and Ordovician periods, a new increase in biodiversity occurred. This event, called the Great Ordovician Biodiversification Event (GOBE), is often viewed as a later stage following the Cambrian explosion. Recent research suggests that the Cambrian explosion and GOBE may not have been two separate events but instead part of one long period of evolutionary change. Studies analyzing data from the Geobiodiversity Database (GBDB) and Paleobiology Database (PBDB) did not find evidence to support the idea that these two events were distinct.
Some scientists have proposed that a period of lower biodiversity, called the Furongian Gap, separated the Cambrian explosion and GOBE. However, studies of fossil sites such as the Guole Konservat-Lagerstätte and similar locations in South China show that the Furongian period was instead marked by rapid changes in life forms. This has made the idea of the Furongian Gap very controversial among researchers.
Uniqueness of the early Cambrian biodiversification
The "Cambrian explosion" can be seen as two stages of animal growth into new environments: first, animals increased in variety as they adapted to the Ediacaran seafloor, and later, in the early Cambrian period, they expanded into the water column. The speed at which new animal types appeared during the Cambrian period was the fastest ever recorded in marine life. This diversification affected all groups of animals for which Cambrian fossils have been found. Later events, like the spread of fish in the Silurian and Devonian periods, involved fewer types of animals and mostly similar body structures. Although the recovery of life after the Permian-Triassic extinction began with as few animal species as during the Cambrian explosion, it created far fewer completely new types of animals.
The event that caused the early Cambrian diversification created many new ecological niches that were previously unavailable. Once these niches were filled, there was little space for such widespread diversification to happen again, because competition was strong in all environments, and existing species usually had an advantage. If these niches had stayed empty, groups of animals would have continued to change and become so different that we would recognize them as separate phyla. When niches are filled, however, species tend to remain similar even after they split apart, because there is less chance for them to develop new lifestyles or body forms.
There were two similar bursts of growth in the evolution of land plants: first, after a history that was not well known, beginning about 450 million years ago, land plants quickly spread and adapted during the Devonian period, about 400 million years ago. Additionally, flowering plants (angiosperms) appeared and rapidly diversified during the Cretaceous period.